join

In std/collections.

fn join<T, U, K, R>(self: Seq<T>, other: Seq<U>, *, left: T -> K, right: U -> K, on: (T, U) -> Bool, into: (T, U) -> R) : Seq<R>
fn join<T, U, K, R>(self: Seq<T>, other: Seq<U>, *, left: T -> K, right: U -> K, into: (T, U) -> R) : Seq<R>
fn join<T, U, R>(self: Seq<T>, other: Seq<U>, *, on: (T, U) -> Bool, into: (T, U) -> R) : Seq<R>

The pairs of an element of the input and an element of other that match, each built by into, ordered by the input element's position, then other's. A pair matches when left(x) == right(y), when on holds for it, or both; a join states at least one condition.